Posted
by
kdawson
on Wednesday January 13, 2010 @02:27AM
from the deepness-in-the-sky dept.

Ben Newman writes "Of all the tech that's come out of CES this week, nothing has gotten the blogosphere more excited then the RCA Airnergy. A lot of people love the thought of an ever-recharging cell phone, and the Airnergy promises to constantly charge its internal battery through 2.4GHz wireless signals. Neat idea, but as some commenters have pointed out the energy just isn't there to make this work — BOTECs for a full charge range from 100 days to 32 years. Plus, don't let the RCA brand fool you into thinking this must be from a legitimate company: RCA hasn't existed as anything more then a licensed brand name for a couple of decades. So what do Slashdotters think — real deal or 21st century hokum?"

If I climb onto the roof of my local shopping mall and stand next to the cell phone mast, maybe it will burn me, but down on the ground it certainly doesn't. I suppose it is about the same as having an electric heater element up on the roof of my shopping mall.

As I said above, that would be the energy of a single photon. Not relevant to energy transmission on the scale we are talking here. Looking up a formula that has energy and wavelength in it is not enough to understand where this formula is actually relevant. Now, if we were talking about single photon detectors, that would be something different. But we are not.

For EM, you really need an antenna that's close to the wavelength, and for 50Hz that's 6000km. For 2.4GHz it's about 12cm.

For induction, frequency affects the overall number of turns required. A 50Hz transformer that copes with 300W is the size of a shoe box, but for a switchmode power supply at 100KHz it's the size of a match box.

Look again into that physics text. Frequency is denoted by "f", not "ny" (no, that is not a lowercase v). The formula you quote relates to the energy of a single photon, no voltage or current in there anywhere and is completely irrelevant to the discussion at hand.

The greek nu is the standard notation for frequency in physics, or at least it generally was when I did that kind of thing. f is sometimes used, though less commonly. However, photons are not relevant for RF - the photon energy is so small that the quantum nature of the radiation is not apparent, and it behaves for all practical purposes as a classical wave.

Much earlier than the mythbusters a german tv-show for kids, the "Sendung mit der Maus" ("program with the mouse") made the point: they held up a neon tube next to a state radio-transmitter and it began to glow. And they explained to the kids, that it will work next to such a high energy radio transmitter, but it is also robbery according to german law. So don't try, kids, unless you don't mind being a thief.

Much earlier than the mythbusters a german tv-show for kids, the "Sendung mit der Maus" ("program with the mouse") made the point: they held up a neon tube next to a state radio-transmitter and it began to glow. And they explained to the kids, that it will work next to such a high energy radio transmitter, but it is also robbery according to german law.

Mythbusters isn't always right. For example, they busted the "myth" of an ancient Greek "Death Ray" by making one that didn't work. Some MIT kids showed it was possible (this was discussed on slashdot last year).

They had busted the "myth" of a sniper shooting another sniper through the second sniper's scope, and the US Army showed they were wrong, by giving them some better ammo. They covered this on the show itself.

I saw the episode you refer to, IIRC they used a device they bought from the internet. Just because that device wouldn't work doesn't mean none would. You should be able to get voltage from stray EMF from your house current; a crystal radio has no power source and is powered only by the transmitter's signal. But it takes tiny amounts of current for headphones to work, a phone takes quite a bit of juice.

So I'm skeptical. I'll believe it when I see one. I do think you could probably make an LCD clock without a power source, you can run an LCD watch from a potato battery.

While I am certainly no degree holder or scholar in this field, I often wondered about the following:

A coil of wire with a current running through it emits a magnetic force. From memory, a magnetic force applied to a coil generates a current. Seeing as the earth is covered in a huge magnetic shell, how come people don't actually generate power this way? Is the magnetic field simply too weak compared to what is needed to generate a current of any value?

It also has to do with a static field vs a moving field. Make a coil of wire, hold a magnet next to it, hook it to a voltmeter. Notice the coil doesn't have any induced voltage until you move the magnet. You can't get any energy out of a static field, no matter the strength of the field.

Nasa HAS tried this.http://en.wikipedia.org/wiki/Electrodynamic_tether [wikipedia.org]You can generate electricity as you move around the earth. Being in orbit, you are going fast enough to make worthwhile magnetic flux, and you are free of air resistance that would keep you from deploying the tether if you were lower in the atmosphere.

trouble is, that earths magnetic field seems to not be as even as we had imagined it, and you tend to get surges that ruin the equipment. (weld the tether reel solid, foul up electronics and my personal favorite, snap the tether like a fuse)

A *changing* magnetic field generates a current. If you just take a coil with some wires attached, and hook up a voltmeter, nothing will happen. Only when you start moving your coil through a magnetic field will you start to see volts. (Earth's field is extremely weak, but with a big coil and a sensitive meter you could see a small current.)

The reason this can't be used for infinite power generation is that the coil will resist movement. Any flow of current generates a magnetic field of its own, and if you do the math, it turns out that the induced current in your coil creates a field in opposition to the field it's moving through. It works against you like a kind of friction, or like air resistance. If you just give the coil an initial kick, it will quickly run down to a stop. In order to generate power you have to keep putting energy into the system.

In other words, you're not draining energy from the magnetic field, you're just converting the kinetic energy you put in.

This is in fact how generators work. A big conductive coil is spun around inside the field of some permanent magnets. If your generator is connected to a water turbine, you're converting the kinetic energy in falling water into the kinetic energy of a spinning coil and thence to electrical energy in a wire.

As others have pointed out, you need a change in the field to generate current, either by moving through it or moving the thing making the field. You can observe this effect on a high speed aircraft, where there is a measurable potential difference between the wingtips as it flies though the earth's magnetic field. It's not really strong enough to extract anything useful out of it - you couldn't hook it up to a motor and power the plane, for instance. You'd do better bolting PV panels on every exposed surfa

A coil of wire with a current running through it emits a magnetic force. From memory, a magnetic force applied to a coil generates a current. Seeing as the earth is covered in a huge magnetic shell, how come people don't actually generate power this way? Is the magnetic field simply too weak compared to what is needed to generate a current of any value?

Perhaps one of you smart folk here might help me out with this little "backburner" thought that I have had for a while?

I remember an episode of Star Trek where the Captain and Spock admire a source of lighting that "produces light, but no heat! - How advanced!" yet, compared to incandescent bulbs, that's an apt description of LED lights, especially those designed for high efficiency [wikipedia.org]!

Let's talk now about Cell Phones - I almost bought a cheapie cell phone for $29 that was about the depth/width/length of a hershey chocolate bar. It sported 2 days of battery life, unlimited text/pic

Now, if you figure that we can improve power conductivity by, oh say, 50%, and can cut power utilization by 100x, (1/10th the amount claimed by Bell Labs) then suddenly, the charge rates from a 150 mw 802.11 radio source 5 meters away actually seems reasonable!

It won't happen today, or tomorrow. But in a few years? Not only possible, but likely!

Not at all. Cellphones need something like 100mW...2W RF output to cut though background static and get a signal to the cell tower. And by conservation of energy that means even if nothing at all besides the RF emitter consumes energy, the power consumption will be at least 100mW...2W.

But it would work, right? It just wouldn't be enough for an indefinite charge. I think it's enough that you extend unplugged lifetime, surely. It's not necessary to have an everlasting charge so much as a longer-lasting charge, offsetting the energy consumption by continual recharging. Isn't that the idea?

I tried it with a 500mW power source on 460MHz using a pair of resonant quarter-wave aerials. At about one metre separation, it was receiving around -6dBm, or about 250uW. So that's ten times the power, ten times closer, on a lower frequency with better propagation. Ten metres away and 50mW would give -26dBm which my meter won't measure, but is one hundredth the power - 2.5uW. Good luck charging a battery with that.

Is totally gonna charge up your battery and run your cell phone for days.

Why not have a phone which works like self winding/powered watches? Something which extracts mechanical energy from body movement. Indeed with a phone such a mechanism could be put entirely inside the "battery" so no need to modify anything else.

The inverse square law and dBm being a logarithmic unit can all go to hell.

You might do better by covering the thing in photovoltaic cells. Given that there are likely to be more visible

You are not considering the possibility that this device is not intended for your average user.Maybe it was envisioned with a more... active crowd in mind.

You know... the kind of people who would find the prospect of running up to one of these [wikipedia.org] a valid possibility.

Also, it would make a GREAT plausible denial device for the active denial system (PDDfADS).Hang one of those around your neck and you can claim that you were just trying to charge your phone (and not the ADS), when you are arrested for whatever act

I'm sure we can look forward to a vigorous debate, where both sides bring up excellent points. I certainly cannot say where the slashdot community will land on this question, and the article certainly doesn't give any hints! Thanks, Ben, for your valuable question, and I hope you find the answers both challenging and enlightening.

Seriously, especially in your case of being "out in the sticks", you'll get maybe -90db of signal, which isn't going to be sufficient to power the radio receiver, much less send the occasional blip back to the tower using the transmitter. You might extend the 2-day battery life of your phone by, possibly, about 10 minutes. And I'm being generous.

I suppose you could leave the charger battery at camp and have enough to charge/power the phone there for occasional visits, but yo

To add to complexity of this, there's multiple devices transmitting all the time.

So we would need to first observe the average of all broadcasts strengths to do more precise calculation.

Tho unless, there's about 1,016,160W transmitter within 10meters (enough to charge blackberry in 1hr), according to your maths this device cannot work, unless it's extremely small powered device, consuming about 1/100th of a blackberry and there's over 100W of broadcasting within 10meters, we start to reach the 1hr ballpark.

This is embarrassing. Embarrassing for Thompson, which owns the RCA brand. Embarrassing for press who took it seriously. Serious career trouble for whomever authorized licensing the name to the clowns behind this.

Oh, it is. If you can live with horrible efficiency, unless very close to the sender. For typical ambient RF, you are lucky to get 1 miliwatt out of this, far less than is usable as a battery charger. This is really basic physics stuff. While the idea is sound, it just does not work with the numbers found in practice.

1) Note that Tesla never got wireless power working. He liked the idea, but he couldn't make it work. Also note that to this day we still don't have it. That should tell you something. The problem, it turns out, is that EM power decays logarithmically with distance. So a little more distance translates to large losses in power. You have a 1 watt transmitter and it is only a few milliwatts when you get a bit away from it. It would be extremely inefficient to transmit power through the air, even ignoring other problems.

2) Tesla was nuts, like "lock him in a soft room" crazy. He was brilliant, don't get me wrong, but he was also crazy. The guy had some really wacky ideas, along with some extremely genius ones. Just because Tesla thought something would work, doesn't mean it would.

"However, Tesla's claims were backed up with documented experimental demonstrations rather than mathematical equations. In the following quotation, Meyl describes one of Tesla's demonstrations and states that Hertz's technology could not have accomplished such a demonstration:

If this technology worked properly instead of most people here assuming that it doesn't it would be huge. The idea of having a few wireless units plugged into home outlets like a Glade air freshener and never having to deal with this charger for this unit and another for another unit and tracking down yet another charge for a unit that only needs charged once a month would get a lot of play.

Not enough energy available. Would probably not even offset self-discharging unless a pretty large antenna is used. You can fake a demo though with a highly directional antenna to beam in a wireless signal. Not realistic at all and inefficient as hell.

They didn't even need to do that for this demo. They "pre-charged" it using WiFi, with no indication of how long it took to charge. They probably had to have the prototype built back in June or July and set it right next to a dedicated access point dialed up to "11" since then to get enough charge to top a Blackberry from 30% to 100%.

If they're really lucky, they'll have the SAME device recharged for CES next year and it can charge a Blackberry from 0% to 100%. They'll have to have 4-5 more access points

Plus, don't let the RCA brand fool you into thinking this must be from a legitimate company: RCA hasn't existed as anything more then a licensed brand name for a couple of decades.

You got that right. Neutron Jack cannibalized RCA in the late 80s, selling the consumer electronics division to Thompson. About 12 years ago, they sold chinese company TCL the right to use the RCA name on TVs and other products.

They ought to replace Nipper with one of those chinese hounds with all the extra folds of skin. HIs master's voice is in chinese.

Nokia proposed a power-harvesting (and power-sipping) handset over the summer last year, to derive its power from cellular signals rather than wi-fi. Although their target amount of 50mW is way off, they claim to have a prototype that can pull in a few milliwatts, which inspired a mixture of scepticism and existential terror from researchers in the field.

Nokia proposed a power-harvesting (and power-sipping) handset over the summer last year, to derive its power from cellular signals rather than wi-fi. Although their target amount of 50mW is way off, they claim to have a prototype that can pull in a few milliwatts, which inspired a mixture of scepticism and existential terror from researchers in the field.

That would be very incompetent researchers then, as the "a few miliwatt" figure is very old and typically found in a classical radio experiment for teenag

The crux is to use "Magnetically coupled resonance" to achieve efficient power transfer to prevent the vast majority of power from being broadcast into space (read wasted) when no receiver is present to absorb it. Unfortunately that very feature seems to severely limit the transmission range.

Doing a simple calc with the above spreadsheet at 2.4 Ghz, 1W of radiated power, patch antenna, and five meters distance from the radiated power source, you would have just 0.040 mW of usable power, and at that, not enough to l

Tesla was a very clever man, but his experiments (tesla coil etc) were based on something quite different than "broadcast energy".

Tesla played with "harmonic" or "tuned" energy, eg take two tuning forks tuned to the same frequency, tap one to set it going, and hold it three inches away from the second one, the second one will start to vibrate, you just transferred energy.

The primary and secondary circuits in a tesla coil works the same way, not with sound, but with tuned electromagnetic force, it is a tuned step up transformer.

The SINGLE wireless power experiment that worked recently worked on the same principle, tuned magnetic coupling.

***This*** device is about simple absorbtion, so yes, it *will* absorb power, and yes it will *charge* a battery, technically speaking, so will your old external TV antenna, satellite dish, ham radio antenna, and indeed how the hell do you think the old crystal / cats whisker radios worked without a battery? It works for RFID too.

But *practically* the rate of "charge" you get out of this is going to be less than the rate of self discharge, even for s single AAA size rechargeable battery.

The physical analogy is a steel plate placed in the bottom of an empty swimming pool with indeed grab water condensation from the air overnight and "charge" the swimming pool with water.

It will NEVER fill the pool though, the self discharge (evaporation) is a faster and more robust process.

It has to apply much more power than that necessary to simply charge the phones. Unless of course you carry the cell tower within 1 foot of the cell phone everywhere you go, otherwise you have to start dealing with inverse square laws and all that nonsense sciencey and mathy stuff.

Multiply it by the hundreds to thousands of cellphones within one cell... can you imagine how much power the cell tower much emit in order to charge all those phones?

FWIW: the article says that the charger makes electricity from "ambient WiFi signals" -- not from the cell tower. Allegedly, at the trade show, "they were able to charge a BlackBerry from 30% to full in about 90 minutes."

Actually, the article says "At CES, the device's battery, which I believe was precharged with Wi-Fi power, was able to charge a BlackBerry from 30% power to full power in about 90 minutes." Note the "which I believe was PRECHARGED" part. So they managed to charge a Blackberry from a pre-charged external battery in 90 minutes. Yay. But they never actually said how long it takes to charge the battery in the Airnergy device via wi-fi signals - probably for a good reason, because that would take probably a coup

Actually, the article says "At CES, the device's battery, which I believe was precharged with Wi-Fi power, was able to charge a BlackBerry from 30% power to full power in about 90 minutes." Note the "which I believe was PRECHARGED" part. So they managed to charge a Blackberry from a pre-charged external battery in 90 minutes. Yay. But they never actually said how long it takes to charge the battery in the Airnergy device via wi-fi signals - probably for a good reason, because that would take probably a coup

Multiply it by the hundreds to thousands of cellphones within one cell... can you imagine how much power the cell tower much emit in order to charge all those phones?

FWIW: the article says that the charger makes electricity from "ambient WiFi signals" -- not from the cell tower. Allegedly, at the trade show, "they were able to charge a BlackBerry from 30% to full in about 90 minutes."

That's amazing, because it takes that long to charge my POS cell from the wall with a... hmmm... ya know, something just doesn't seem right here...

They make kinetic chargers and stitch-on solar panels that would both yield orders of magnitude more power.

The AA batteries you mention are also a superior alternative. They would hold more power than this thing could collect in months, and are disposable and cheaply replaced, and are actually better for the environment. This gizmo might replace AT BEST about 10 AA batteries in its lifetime, but it's far harder to produce, and has a heavy-metal rechargeable battery and features a lot more gimcrackery insi